A number of possible methods are available for thickness monitoring. Two of the best known approaches use ultrasonic thickness ranging or nuclear-radiation attenuation measurements. Problems with the ultrasonic approach are the need for a good liquid coupling between the transducer and the inspected materials, as well as variations of the acoustic velocity and a high attenuation in the media under consideration. As to the X-ray technique, problems are related to a limited response speed, reduced transverse resolution, high price and safety questions. Other methods using tactile sensors or microwaves do not possess the required resolution.
Optical techniques are the most widely used methods for three-dimensional surface profiling. Apart from the time-of-flight ranging approach which is presently limited to depth resolutions of the order of 1 cm, optical techniques are essentially based on triangulation. This method consists in the projection of a light beam forming a luminous spot on the surface whose position is to be measured. By viewing the projected spot from an angle, one can evaluate the depth of the surface from the apparent position of the spot image.
To produce a three dimensional profile of the workpiece, a number of triangulation units can be aligned along the longitudinal axis of the elongated workpiece to collect top and bottom surface height information at specified spatial intervals while the object is moved at uniform speed in the transverse direction, as it is described in U.S. Pat. No. 3,671,726 by Kerr. A single laser source in conjunction with a number of beam splitters can be used to obtain the required number of projected beams.
The spatial resolution which can be obtained in this case along the longitudinal direction is limited to the spatial interval between projected spots which must be at least equal to the size of the projection and detection units. This results in a relatively coarse sampling distance along the longitudinal direction. Furthermore, each detection unit including a number of optical and electronic components is devoted to the evaluation of a single spot position. A large number of units must consequently by used to scan a relatively long workpiece, leading to a relatively high cost of the overall system.
A different approach is described in U.S. Pat. No. 4,188,544 by Chasson. In this case a number of light stripes is projected across the transverse direction of the workpiece. Each stripe is imaged through a separate mirror on a specific portion of the sensitive area of a two-dimensional video camera. Hence, only one viewing device is required to view a multitude of projected stripes.
This second approach presents a number of drawbacks. First, a single camera must be focused to view several stripes situated at different distances from the camera. Parts of the image seen by the camera will thus be out of focus, unless the camera aperture is strongly reduced at the expense of the overall image brightness. The laser power must be spread over a stripe rather than being concentrated on single spots as in the case mentioned above. The laser power must be correspondingly increased to reach a sufficiently high light intensity, thus raising eye safety objections. Finally, and most importantly, this approach is severely limited in terms of speed. In a typical 480.times.600 pixel CCD camera, the time required to scan the full two-dimensional image is normally 1/30 second. Substantial workpiece displacement may take place in several industrial processes during such a period, resulting in image blurring. To avoid such problems one must resort to complex mechanical systems or to high-peak-power pulsed laser beams, further increasing eye safety concerns. Moreover, the requirement to process the huge amount of nearly 3.multidot.10.sup.5 bytes of image information during each 1/30 second is very demanding in terms of computing power and overall cost.
It is an object of the present invention to provide an apparatus and a method for monitoring the position of a surface of a moving workpiece in a safe and efficient manner.